The present invention relates to a system and a method for controlling airflow in a multiple bed desiccant drying system.
Multiple desiccant bed systems are known for drying, for example, a synthetic plastic material, in which a moisture-laden gas stream is formed as the exit gas from a hopper in which plastic granules are dried by a stream of drying air. During an adsorption phase, the exit gas from the hopper is conducted through one or more drying vessels filled with an adsorption medium, whereby the adsorption medium extracts the moisture from the gas so that the resulting dry gas can be used again as a drying gas for drying plastic granules. Existing multiple desiccant bed systems designs use desiccant beds or moving rotors containing desiccant, with the individual desiccant elements being arranged in a parallel grouping, such that the dry gas effluent is a combination of the gas dried thru out the system. As a result, the quality of the dry gas effluent can vary, as variations in water loading in individual desiccant beds may result in a portion of the effluent gas having a higher than desired water content, which in turn leads to undesired increases in the water content of the recombined dry gas volumes.
When the adsorption medium in a drying vessel is saturated with moisture, the drying vessel is typically transferred to a regeneration phase in which heated ambient air is conducted through the adsorption medium, which takes up and carries away the moisture which was adsorbed therein. By using a multiple bed system, the drying process can be continued essentially without interruption, by utilizing one or a portion of the beds for adsorption, while simultaneously regenerating other beds, by appropriately channeling the process air flow. Even with this regeneration process, however, in parallel-grouped multiple desiccant bed systems the dry effluent gas remains a mixture of gas with varying moisture content exiting from the desiccant beds that remain on-line as other beds are regenerated.
Thus, there is a need for an improved system and method for providing a dry gas effluent from a multiple desiccant bed system which has a consistently low moisture content.
The present invention addresses the foregoing problems by providing a multiple desiccant bed system in which three or more desiccant chambers, each containing its own adsorbing desiccant bed along with a heater element for regeneration of the individual desiccant bed, are operated in series, with the ability to remove at least one of the desiccant beds from the gas flow circuit during bed regeneration while gas flow continues in the remaining desiccant bed(s). Further, the system is arranged to permit the desiccant bed most recently regenerated, and thus having the lowest bed moisture loading and producing the driest gas effluent, to be located in the finishing position at the end of the series of desiccant beds. The present invention thereby provides a better quality of dry gas effluent, as the dry gas always will leave a more recently regenerated desiccant bed than in current parallel flow desiccant bed designs.
In an embodiment of the present invention, multiple desiccant beds are arranged about a vertical axis, so that the air flow will pass horizontally through the desiccant beads in the beds. This arrangement has the benefit of enabling the top of the bed to have a reservoir of desiccant above the working material to allow for settling or attrition. The desiccant beds can be arranged in a variety of configurations, including a square or circular pattern, depending on how many individual beds are required.
Above the bed assembly there is provided an inlet plenum for wet gas influent. A similar outlet plenum is provided below the bed assembly for dry air discharge. Note that the references to a top inlet and a bottom discharge are illustrative only, as the whole arrangement can be built in any number of ways as long as the beds are interconnected by adjoining common partitions or ducting.
The desiccant bed chambers in this embodiment are arranged such that interconnecting ducts permit gas flow in series between the chambers, with a loop duct re-routing air flow from the bed at the end of the series back to the first bed in the series. Each desiccant bed chamber is provided with a connection to a common wet gas inlet plenum. The inlet plenum is shared with the other beds. Each bed chamber also has a connection to a common dry gas outlet plenum shared with the other beds. The gas flow between the inlet and outlet plenums and between the desiccant bed chambers is controlled by a plurality of three-way valve units, with one three-way valve being associated with each chamber. In addition, each chamber is provided a one-way valve through which air from outside the system, such as room air, may be introduced to the chamber during bed regeneration. The outside air is drawn into the chamber by suction generated by a regeneration blower, which advantageously applies suction to the chamber through a port and valve arrangement integrated into each three-way valve unit valve actuator.
The method employed to operate this system is as follows. There are three individual positions that will provide control of the adsorption gas circuit through the system. The first valve position opens lifts a first valve element, such as a valve disc, off its seat, allowing the incoming wet gas to pass from the inlet plenum into the desiccant system. After passing though the first desiccant bed, the gas passes into the next desiccant bed chamber by passing through a second valve seat of the next chamber's valve unit (this seat may also be referred to as a “pass-through port”), which is opened by lifting a second valve element off the second valve seat. The first valve seat of the second chamber's valve unit remains closed, so that the wet gas in the inlet plenum cannot enter the second desiccant bed chamber except from the first chamber through the second valve seat. This valve position will be used for all desiccant beds that are not in one of the other defined positions.
After entering the second desiccant bed chamber though this “pass through” opening, the gas passes through the second desiccant bed for further drying. The gas then may subsequently pass through as many desiccant bed chamber stages as desired. At the last chamber in the desired flow path, instead of directing the gas through the second seat of the next chamber's valve unit, the next chamber's valve unit is positioned such that the second seat remains closed and a third valve element is lifted off a third valve seat located in the previous chamber. This third valve seat in the previous chamber is a port connecting the chamber to the dry gas outlet plenum, and therefore the dried gas flows from the last, or finishing, desiccant bed chamber into the dry gas outlet plenum. The dry gas may then be extracted from the plenum for use elsewhere. In the event the finishing chamber is also the physically last chamber in the multiple desiccant bed unit, the outlet side of its bed is connected via the return loop duct which directs the gas flow into the inlet side of the physically first desiccant bed chamber.
The desiccant bed chamber having its valve unit in the third position, i.e., with its first and second valve seats closed, is isolated from the gas drying flow circuit. Accordingly, while other desiccant bed chambers are being used for wet gas drying, the “off-line” chamber may be regenerated as follows. A regeneration control valve integrated into the chamber's valve unit is operated to open a regeneration port to the chamber. A regeneration blower may then apply suction to the chamber to draw regenerating air across the desiccant bed. When suction is applied to the regeneration port, a one-way check valve in the wall of the chamber is drawn open, allowing outside air to enter the chamber to flow across the desiccant bed. One of ordinary skill will be able to readily envision other approaches to obtaining the desired outside air flow across the desiccant bed, such as by supplying compressed air to the chamber, and opening a port to the outside on the other side of the bed to permit the regenerating air to escape the chamber.
Once a desiccant bed has been regenerated, the valve units are preferably repositioned so that the most recently regenerated desiccant bed chamber is made the finishing chamber, with its connection to the dry gas outlet plenum being open. This ensures the gas leaving the multiple desiccant bed system will always achieve the lowest possible gas moisture content.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying figures.